Current confinement structures are used in semiconductor lasers to provide low threshold and high efficiency. One typical structure is formed by growing an oxide surface layer, then, using mask technology, etching away selected portions of the oxide. The oxide material is insulative so current flows only into those areas of the underlying laser structure where the oxide layer has been removed. Another typical structure is formed by implanting protons into selected areas of the semiconductor laser surface. The protons damage the semiconductor crystal lattice making those areas insulative, just like the oxide layer.
In U.S. Pat. No. 3,936,322, Blum et al. describe a method where oxygen ions are implanted in regions on each side of the light emitting active region to make such implanted side areas semi-insulating or very highly resistive to electric current. The ions form electron traps which are effective at making the implanted regions semi-insulative when the oxygen density exceeds 10.sup.17 /cm.sup.3. Implantation damage can be removed by annealing at 600.degree. C. for 30 minutes.
Several techniques use multiple growths to produce current blocking layers below the laser's surface so as to confine current to particular regions of the laser. For example, a p-type blocking layer can be grown on an n-type substrate and openings made in selected areas of the p-type blocking layer where current flow is desired. Next, an n-type cladding layer, active region, p-type cladding layer and p-type cap layer may be grown to complete the heterostructure. The reverse biased p-n junction between the n-type cladding layer and the p-type blocking layer has a higher turn-on voltage and higher threshold than the n-n interface between n-type cladding layer and n-type substrate in the areas opened in the blocking layer, so current is effectively confined.
In U.S. Pat. No. 4,783,425, Fukuzawa et al. describe a fabrication process for a laser heterostructure in which part of the p-AlGaAs upper cladding layer of the heterostructure is deposited, n-GaAs current confinement layer regions are formed on the cladding layer and the remainder of the p-AlGaAs upper cladding layer is deposited over the first cladding part and the confinement regions. The current confinement layer regions are formed either by an epitaxial layer deposition followed by a selective etch of the n-GaAs or by a selective epitaxial layer deposition of the n-GaAs, using a SiO.sub.2 mask layer wherever deposition is unwanted. Since any exposed AlGaAs readily oxidizes, forming an unstable degradation layer, a thin undoped GaAs "protecting layer" is formed over the first part of the cladding layer prior to forming the current confinement layer regions. The thin protecting layer is later thermally interdiffused with the cladding material with the aid of disorder inducing zinc dopant in the overlying AlGaAs cladding material.
Other techniques use diffusion of n- or p-type dopants rather than etching to "open" up current blocking layers of opposite conductivity type. For example, a laser heterostructure comprising n-type substrate and cladding layers, an active region, and p-type cladding and cap layers may be grown with an n-type current blocking layer included either at the surface above the cap layer or below the surface between the cladding and cap layers or even dividing the p-type cladding layer in two. Next, a p-type dopant, such as zinc, is surface implanted in selected locations and thermally diffused downward through the n-type current blocking layer. The diffused dopant alters the conductivity type of the blocking layer in selected locations to p-type allowing current to flow through these locations but not through the unaltered areas of the blocking layer.
N. Bar-Chaim, et al. in "Be-Implanted (GaAl)As Stripe Geometry Lasers", Applied Physics Letters, vol. 36, no. 4, 15 Feb. 1980, pp. 233-235, describe another diffusion method for obtaining current confinement in which a GaAs/GaAlAs heterostructure is grown which is all n-type. Next, beryllium ions (a p-type dopant) are surface implanted in stripes. The wafer is then annealed for 40 minutes at 800.degree. C. This results in diffusion of the implanted stripe down to the GaAs active region creating a p-n junction for active gain. The structure prevents current spreading around the stripe while limiting the gain region to the stripe width and to small minority-carrier current tails on either side of the stripe.
In U.S. Pat. No. 4,532,700, Kinney et al. describe a method for electrically isolating semiconductor structures in which a p-type silicon body is ion implanted with N-type ions to form N-type buried regions. N-type surface regions may also be formed. The N-type regions, under certain anodic etching conditions, etch slower than the surrounding silicon material. The structure is anodically etched to selectively convert all but the formed N-type regions into porous silicon, which is then converted into an electrically insulative, silicon dioxide dielectric by oxidation. The structure is finally annealed to densify the oxidized material.
Each of the known methods are complex and require many steps, such as growth, masking, photolithography, etching, mask removal, growth of next layer, etc. Further, some of the resulting barrier layer structures are not perfect barriers and allow some current leakage due to tunneling.
It is an object of the present invention to provide a simpler process of forming current blocking layers in a semiconductor laser or laser array, and a laser structure so formed.